Why do people leave?

Those two probably left for a quickie

Boredom!
Why sit through anything if you think you can enjoy your time more somewhere else. I'd rather these people leave than grumble, fidget, etc and distract me!

It's also very possible/likely that they moved to different seats for the 2nd act. Especially for a show like Curious Incident where it's better viewed from head on rather than from the side.

Also, how many shows "come together" later in Act 2? I've gone through so many entertainment media where something gets better that I'd try to give it a real honest try (like a TV series improving greatly halfway through a season, or in the second season, etc.).

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Things are not always what they seem, either. I was surprised when a woman next to me didn't come back for a second act recently. But then I happened to see her exit from the theatre after the show. Clearly she had just moved to a different seat.

I have been known to have a "bathroom emergency" in the middle of the first act. I suppose because I then stay in the back for the rest of the act, then those around me had assumed I had left.

Some shows also end up not being what you were hoping for, despite doing research or maybe seeing something that left you curious. Like I didn't hate Phantom of the Opera, but I ended up not as excited about it as I had before seeing it. I don't think I could sit through it again.

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"why not do better research before to see if it is something that might appeal to you?"

I try and do as little research as possible in advance, since I don't really want to know all of the details of the show. So, if a show has won awards, or I know someone in the cast, or has some intriguing element, I'll likely buy a ticket in advance. If I'm unclear, I'll wait until first preview, and then not read the reviews in their entirety, but just read the beginnings/ends for whether it sounds worthwhile.

A lot of people seem to want too much information these days. Movie trailers show you pretty spoiler elements, not to mention people describing the detailed story of the Broadway show they're sitting there to see a few minutes before it about to happen... but I'm not a fan of knowing a lot in advance.

That said, sometimes you'll still end up in a show that doesn't work for you. But I never leave at that point, I just ride it out.

As for upgrading your seats, I don't really care to make that a separate thread, since it boils down to "who cares?" You don't think people should, other people do it and are fine, so... you're right, and they're right, especially since many ushers have told me exactly where I could move to, if I wanted.

I've never done it, but frankly, if someone doesn't want to be there, I'd rather they leave than sit through the second act complaining, being distracting or rude to the rest of the audience and the actors. So, they were doing everyone else a favor (and trust me, I've been to shows where I seriously wish people around me had left at intermission).

I have left shows only three times in 40 plus years!, and I wish I had left time and another dozen times at least.

The ones I left were two of the worst musicals ever created, Jimmy and Dr. Jazz. Hated the latter so much that we decided we'd had enough when two morbidly obese women in totally sheer nighties did a suggestive dance (I was probably 25 at the time and not prude...just grossed out) probably no more than 40 minutes into the show. We had been totally bored for the entire time. The only play I left had received great reviews and was actually visually stunning at times...Ulysses in Nighttown; we were so bored out of our minds that we just left. As I recall, there was no intermission, so it was a little awkward since we were in the middle of the row, but you only live once.

Things I wish I had left (the ones i can remember):Copperfield, The Leaf People, Brief Lives (a one person show that also got great reviews and which put me to sleep), Marco Polo Sings a Solo (great cast, godawful piece of garbage hiding under the term of absurdism), The Ambassadors, Waltz of the Toreadors (Circle in the Square), Cats (one great song does not a show make...I hated all but 6 - 8 minutes), What Makes Sammy Run, No Man's Land (with Gielgud and Richardson, no less...I concluded that I hate Pinter with that one).

My criteria in the final analysis: am I bored out of my mind and not paying attention at all because it is so bad. I paid my money, I had better things to do that sit through really boring stuff, and I had no obligation to anyone to sit any longer.

I know a friend who's not into theater, but her and her husband purchased tickets to the Lion King to give it a shot. Both didn't last the first act and left before intermission to head to a sports bar. They were expensive seats too. Said it really wasn't for them. I don't understand it, but I guess they would rather spend time elsewhere!

More often I will stick it out to the end even if I am having a terrible time. It seems to be the most respectful thing to do for the artists at work here. A couple of times, I've left because I've been so embarrassed for the actors onstage and it was too painful to remain.

At a performance of Lady Day a few weeks back, my wife was seated next to a man who quietly snored through much of the show. His snoring crescendoed as the show did in the final number. I really, really wish he had just stepped out. I'd have gladly stood for him to pass rather than snoring through an experience I really treasured.

Years ago I would always stay no matter what. Today I give myself permission to leave a tortuously boring or bad show. I still rarely leave but I rather not waste any more of my valuable time by staying.

Sometimes the seat location will help me stay or go too. I left "Leap of Faith" because I was not having any fun whatsoever and was stuck in the boondock seats at the St. James. Had I been in a good seat in the orchestra, I probably would have stayed simply to root for Raul up close. This is what happened at "The Anarchist." I wanted so badly to leave but had a decent seat so just kept focusing on and praying for Patti.